1. Field of the Invention
The present invention relates to a technical field for creating a halftone dot image which represents an original image of gray scale.
2. Description of the Background Art
To create data for printing plate from data of an original image of gray scale (i.e., continuous tone), halftone dots are used in many cases. In a generally-used AM (Amplitude Modulated) screening, grayscale representation is made by changing the size of dots, instead of changing the number of dots constituting a halftone dot image. On the other hand, in an FM (Frequency Modulated) screening, grayscale representation is made by changing the number of dots of certain size, which are arranged appropriately (in a random fashion without extreme local difference in density). Further, as disclosed in Japanese Patent Publication No. 3427026 (Document 1), grayscale representation is made by changing the size of dots which are arranged in a random fashion.
In the AM screening, if the original image has periodicity, there may arise problems of causing an interference moire or causing an overlapping moire in multicolor printing. In general, if the halftone dots for two color components are superposed with their arrangement directions of dots being different at an angle of 30 (or 60) degrees, the overlapping moire is made less obvious. However, superposing the halftone dots for three color components, i.e., cyan, magenta and black, with their arrangement directions of halftone dots being different at an angle of, e.g., 15, 45 and 75 degrees from the horizontal direction causes a rosette pattern (moire pattern). In AM halftone dots, the overlapping moire becomes obvious as the difference in arrangement direction of dots is smaller. For example, if the difference in arrangement direction is 15 degrees, a 15 degree moire appears.
In FM halftone dots or halftone dots generated by the method disclosed in Document 1, no interference moire nor overlapping moire appears since the distances of dots are not uniform and the arrangement of dots has no directivity. In multicolor printing, however, the above halftone dots have tendency of making graininess more obvious.
It is thought that the graininess in a multicolor halftone dot image in which halftone dots for these color components arranged in a random fashion are superposed is caused by variation in overlapping dots due to the arrangement of dots without consideration of superposition of images of a plurality of color components (color separations), which is recognized as patterns of low frequency or color irregularity. As measures against the graininess, it can be thought that the density of dots should be made higher (the screen ruling should be made higher), but a writing with extremely fine dots may cause deterioration in stability of printing and make a swath pattern (a pattern appearing in a scan direction) of an output device more obvious in plate making. Then, it is thought preferable to reduce the graininess by preventing local overlapping of halftone dots for these color components.
For example, Published Japanese Translation of a PCT Application No. 2002-540735 (Document 2) discloses a method for making an effect of the positional difference in multicolor printing less obvious, where dots are arranged in a specific direction with pseudo-randomness obtained by using mathematical expressions and the directivity of dot arrangement is changed by colors. Japanese Patent Publication No. 2905106 (Document 3) discloses a method of reducing the graininess by controlling the arrangement of dots so that the overlapping of dots which are randomly arranged in the FM halftone dot should be reduced to as less as possible.
In the method of Document 2, since the pseudo-randomness is obtained by using mathematical expressions, there is a possibility that the interference moire may be caused if the original image has periodicity. On the other hand, in the method of reducing the graininess by controlling the arrangement of dots, an image with much less graininess can be obtained if registration (superposition of images of some color components) is correctly made, but there is a possibility that unexpected graininess may be caused by a slight misregistration which usually occurs in an actual printing. In other words, in this method of controlling the arrangement of dots, it is indispensable to ensure a strictly-correct registration and therefore the manufacturing cost of a printing apparatus disadvantageously increases.
The unnatural nonuniformity, such as graininess, printing irregularity, swath patterns or interference moire, which appears in a halftone dot image is liable to become more obvious in color separation of black than in color separations of cyan, magenta and yellow.